1. Field of the Invention
The present invention relates to a communication apparatus used in a communication system having a plurality of communication terminals connected to at least one line.
2. Related Background Art
As the communication system of this type, an ISDN (integrated services digital network) has been known. Up to eight terminals can be connected to the ISDN line. The ISDN line has three communication channels, that is, two data channels (B channels) and one control channel (D channel) and up to eight channels are connected to the line. When the communication channel is not busy, it can communicate in response to a communication request from a terminal.
In the ISDN, a common address which is common to all terminals and sub-addresses given to respective terminals are used. When terminals are to be called, if only the common address is designated, all terminals are called and one of them is connected. If a sub-address is designated in addition to the common address, only the terminal of the designated address is called.
Where a small number of lines are shared by a plurality of terminals like in the ISDN system, a terminal may not communicate if the line is busy. Whether it can communicate or not is not known until it issues a communication request.
In the ISDN system, if the terminal is designated by the sub-address, only the designated terminal is called. If the terminal corresponding to the sub-address is busy or no such terminal exists, an operator who tries to connect to the line must again designate the common address and recall.